Optical disk apparatus with a plurality of heads

ABSTRACT

An optical disk apparatus comprises a plurality of heads for performing erasure of information from, record of information on, and reproduction of information from an optical disk. Each of the heads is movable in transverse to tracks on the optical disk for seek movement. Head drive system is provided for driving the heads so as to move the heads to a target track on the disk. The head drive system is adapted to apply for the seek movements of the heads a first drive force and a second drive force which is smaller than the first drive force to each of the heads in such a manner that when one of the heads is driven by the first drive force, the other heads can be driven by the second drive force only.

BACKGROUND OF THE INVENTION

The present invention relates generally to an optical disk system. Moreparticularly, the invention relates to a head drive system forperforming seek movement of each of a plurality of head for erasinginformation from, recording information on and/or reproducinginformation from an optical disk such as magnet optical disk.

In an exemplary optical disk apparatus using an erasable optical disksuch as magnet optical disk, a head for erasing information from,recording information on and/or reproducing information from the opticaldisk is driven by a head drive motor for its seek movement. Generally,drive force is applied to the head in the form of a triangular pattern.Namely, the head is first accelerated at a constant acceleration α andthen decelerated a the constant acceleration α. For this purpose,eclectic current i is supplied from a source to the drive motor througha power amplifier under fixed voltage V.

In this case, when the weight of the head is given as m, the drive forceF for moving the head is represented as F=m α. Since the acceleration αis in proportion to the current i, the drive force F is also inproportion to the current i. Further, since the voltage V is fixed, anelectric power P required to drive the head is represented as P=Vi.

Recently, an optical disk system which can perform erasure, record orreproduction of information by using a plurality of heads has beenproposed. In this kind of optical disk system, it is easy tosimultaneously drive all the heads. However, in this case, an extremelylarge power will be required. In many cases, the system is designed tohave an upper limit of a maximum power consumption. Accordingly, whenall the heads are simultaneously driven, the power consumption requiredto drive the heads is apt to exceed a maximum power consumption,resulting in a breakdown of the system.

On the other hand, when the heads are successively driven in such amanner that after seek movement of one of the heads has been completed,seek movement of the next head is started, a power consumption requiredto drive the heads can be decreased. However, in this case, when seekdistance is relatively long, an overall seek time required to completeall the seek movements of the heads will exceed a permissible time-lagbetween information recording/reproducing operations effected by theseparated heads.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anoptical disk apparatus which makes it possible to perform seek movementsof a plurality of heads at a minimal power consumption withoutincreasing excessively an overall seek time of the heads.

The object of the invention can be achieved by an optical disk apparatuscomprising: a plurality of heads for performing erasure of informationfrom, record of information on, and reproduction of information from anoptical disk, each of the heads being movable in transverse to tracks onthe optical disk for seek movement; means for driving the heads so as tomove the heads to a target track on the disk, the driving means beingadapted to apply for the seek movements of the heads a first drive forceand a second drive force which is smaller than the first drive force toeach of the heads in such a manner that when one of the heads is drivenby the first drive force, the other heads can be driven by the seconddrive force only.

In the above-mentioned optical disk apparatus, when one of the heads isdriven by the first drive force, the other heads can be driven by thesecond drive force which is smaller than the first drive force.Accordingly, electric power consumption required to drive the the headscan be reduced. Further, according to the above-mentioned construction,seek movements of the heads can be partially overlapped in time witheach other. Accordingly, overall seek time required for completing allof seek movements of the heads can be reduced without increasingelectric power consumption.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments of the present invention as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical plan view showing, as an example, arrangementsof two heads of an optical disk apparatus according to the presentinvention;

FIGS. 2 to 6 are velocity diagrams for explaining various head driveoperations to be effected under various conditions, respectively,according to the invention;

FIG. 7 is a view showing various conditions for the head driveoperations according to one and another embodiments of the invention;

FIG. 8 is a block diagram of electronic circuitry of the optical diskapparatus constructed in accordance with the invention;

FIG. 9 is a timing chart of input/output signals generated in thecircuitry shown in FIG. 8;

FIGS. 10 to 12 are flowcharts illustrating various operations executedby a central processing unit (CPU) of the circuitry; and

FIGS. 13A and 13B is a diagram illustrating head drive characteristicsof the optical disk apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is schematically shown a part of an opticaldisk apparatus to which the present invention is embodied. In theillustrated apparatus, information can be erased from, recorded on, orreproduced from an optical disk 13 at each of recording tracks thereonby using a plurality of heads. Although in the illustrated embodiment,the optical disk apparatus comprises two heads 11 and 12, it may includethree or more heads.

In the illustrated embodiment, the head 11 is used to erase oldinformation from the disk 13 at a selected or target track as well as torecord new information on the disk 13 at the selected track, while thehead 12 is used to reproduce the information from the track on the disk13. The heads 11 and 12 are movable across a recording surface of theoptical disk 13 along different radial directions of the disk 13,respectively, which are at an acute angle θ with respect to each other,as shown in FIG. 1. Drive motors (not shown) are provided for performingseek movements of the heads 11 and 12, respectively, to a selected trackon the disk 13 at which erasing, recording or reproducing of informationis to be performed. The optical disk 13 can be rotated by a spindlemotor (not shown) about its center axis in a direction of the arrow Rshown in FIG. 1, and thus, the head 11 takes a position behind the head12 in the direction of rotation of the disk 13.

In the case that for example, erasure of old information from a selectedtrack on the disk 13, record of new information on the selected trackfrom which old information has been erased, and reproduction of the newinformation from the selected track for the check of the new informationrecorded thereon are to be successively performed, the heads 11 and 12are moved to the selected track on the disk 13. After arriving at theselected track, the head 11 is first used to erase old information fromthe selected track and is then used to record new information on theselected track, and thereafter, the head 12 is used to reproduce the newinformation from the selected track for the check of the new informationrecorded thereon.

In the case that for example, the speed of rotation of the optical disk13 is 4320 rpm, one-rotation time D of the disk is about 14 msec, andtime interval d from the time when record of information is performed onthe track by the head 11 to the time when reproduction of theinformation recorded is performed on the track by the head 12 for thecheck of the information recorded thereon is ordinarily several microseconds (d=14×θ/360). Therefore, a time-lag between the times ofseek-ending of the heads 11 and 12 may occur as long as it remainswithin the range of the above-mentioned time interval. Since the heads11 and 12 are moved to the same selected track, seek distances of theheads 11 and 12 are equal to each other. When one of the heads 11 and 12is driven at a constant acceleration α for its accelerated ordecelerated motion, drive force for accelerating or decelerating theother head at the constant acceleration α is not supplied to the otherhead, so that electric power consumption required for both the seekmovements of the heads 11 and 12 can be reduced. Further, in the casethat seek distances of the heads 11 and 12 are not so short, movingtimes of the heads 11 and 12 are partially overlapped to each otherwithout any overlapping of accelerated/decelerated motions of the heads11 and 12. Accordingly, overall seek time required for performing seekmovements of the heads 11 and 12 can be reduced without increasingelectric power consumption.

Explanation will now be made in more detail as to various seek drivecharacteristics of the optical disk apparatus according to oneembodiment of the invention with reference to FIGS. 2 to 7.

I. In the case that the seek distances of the heads 11 and 12 arerelatively short, driving characteristics of a triangular pattern aresuccessively applied to each of the heads 11 and 12, as shown in FIG. 2.That is, after the seek movement of the head 11 according to thetriangular drive characteristics has been completed, the seek movementof the head 12 according to the triangular drive characteristics isstarted.

In this case, when seek time of each of the heads 11 and 12 is indicatedat t, a seek distance x of each of the heads 11 and 12 can berepresented as follows. ##EQU1## Therefore, the seek time t can berepresented as follows. ##EQU2## When a permissible time-lag between thetimes of seek-ending of the heads 11 and 12 is indicated at Td, it isrequired that t≦Td, and thus, the following condition has to besatisfied. ##EQU3##

II. In the case that the seek distance of each of the heads 11 and 12 isslightly longer than the above-described distance, a time-lag betweenthe times of seek-ending of the heads 11 and 12 will becomeimpermissible when the seek distance x becomes ##EQU4## Therefore, inthis embodiment, driving characteristics of a trapezoidal pattern areapplied to the head 11 for the seek movement, as shown in FIG. 3. Thatis, after having been accelerated at a constant acceleration α until apredetermined time t₁, the head 11 is moved at a constant speed until apredetermined time t₂ and is then decelerated at the constantacceleration α. On the other hand, in this case, the head 12 isaccelerated at the constant acceleration α between the times t₁ and ₂and is then moved at a constant speed during deceleration of the head11. Thereafter, the head 12 is accelerated again at the constantacceleration α and is then decelerated again at the constantacceleration α.

In this case, when the accelerated/decelerated ratio of the head 11,i.e., the ratio a total distance of accelerated or decelerated movementsof the head 11 to the seek distance x, is indicated at β, the distanceof movement of the head 11 from the time of seek-starting of the head 11to the time t₁ becomes β x/2, while the distance of movement of the head11 between the times t₁ and t₂ becomes (1-β)x, and the distance ofmovement of the head 11 from the time t₂ to the time of seek-ending ofthe head 11 becomes β x/2. Therefore, the following relationship isheld. ##EQU5## Thus, the time t₁ can be represented as ##EQU6## On theother hand, when it is assumed that t'=t₁ -t₂, the followingrelationship is held.

    (1-β)x=t'·αt.sub.1                     (5)

From equations (4) and (5) the time t' can be represented as ##EQU7##Therefore, the following condition according to equations (4) and (6)has to be satisfied. ##EQU8## At the limit of the condition of formula(7), driving characteristics having a trapezoidal pattern shown in FIG.4 must be applied to each of the heads 11 and 12, in which x=2 αTd².Therefore, in order to ensure that the time-lag between the times ofseek-ending of the heads 11 and 12 is made equal to the permissibletime-lag Td under the condition of αTd/4≦2 α≦2 α Td², the drivendistance ratio β is required to satisfy the condition of formula (7).

III. In the case that the seek distance is more longer than theabove-described distance, when the seek distance x becomes x≧2 αTd², thedriven distance ratio β of the head 11 has to become smaller than thatof the head 12. In this case, therefore, driving characteristics of atrapezoidal pattern are applied to the head 11, as shown in FIG. 5. Thatis, after having been accelerated at a constant acceleration α, the head11 is moved at a constant speed and is then decelerated at the constantacceleration α. On the other hand, in this case, the head 12 isaccelerated at the constant acceleration α from the time ofacceleration-ending of the head 11 to time t₀ and is then decelerated atthe constant acceleration α. Thereafter, the head 12 is moved at aconstant speed during decelerated motion of the head 11 and is thendecelerated at the constant acceleration α.

In FIG. 8, the seek distance of the head 12 is represented by an area Saof the region which is bounded by the one-doted lines and which iscomposed of a shadowed portion and the rest triangular portion which, inturn, represents the seek distance x of the head 12, i.e., the sum ofdistances of accelerated or decelerated sections in the overall seekmovement of the head 12. An area Sb of the shadowed portion can berepresented as ##EQU9## in order to obtain an area Sc of the triangularportion, the time t₀ will now be found. At the right side of a point ofthe time t₀, the triangular portion has a straight line passing througha point which can be represented by ##EQU10## Thus, when the straightline of the triangular portion at at the right side of the point of thetime t₀ is assumed as v=-αt+b, the following equation can be held.##EQU11## Therefore, the straight line of the triangular portion at atthe right side of the point of the time T₀ can be found as ##EQU12##Next, when it is assumed that v(t₀)=α(t₀), the time t₀ can berepresented as ##EQU13## According to equations (8) and (11), the seekdistance x can be represented as ##EQU14## Therefore, when the value ofdriven distance ratio β is decided in accordance with the value of seekdistance x so as to satisfy the condition of equation (12), the time-lagbetween the times of seek-ending of the heads 11 and 12 always becomesequal to the permissible time-lag Td.

The above-described condition of the driven distance ratio β in each ofthe cases of items (I) and (II) can provide optimum mode of drivingcharacteristics to the heads 11 and 12 which make it possible tominimize the seek time of the head 11 while restricting the time-lagbetween the times of seek-ending of the heads 11 and 12 to be equal tothe permissible time-lag Td. However, according to the presentinvention, more simplified driving characteristics may be applied to theheads 11 and 12. For example, in the case of item (II), drivingcharacteristics of a trapezoidal pattern may be applied, as a simplifiedmode, to the head 12 by fixing the driven distance ratio β to β=0.5.Also in the case of item (III), driving characteristics of a trapezoidalpattern may be applied, as a simplified mode, to the head 12, as shownin FIG. 6, in which maximum speed of the head 12 is flattened incorrespondence to that shown in FIG. 4. In this case, from βx=αTd², thedriven distance ratio β is decided so as to satisfy β=αTd² /x.

FIG. 7 shows the list of relationships between x and β in theabove-described optimum and simplified modes of head drivingcharacteristics.

FIG. 8 illustrates an example of electronic circuitry for driving theheads 11 and 12, and FIG. 9 is a timing chart of input/output signalsgenerated in the circuitry shown in FIG. 8.

Referring to FIGS. 8 and 9, a micro computer (CPU) 14 executes apredetermined process on the basis of commands from a host computer 15and generate head drive signals which are sent to digital to analog(D/A) converters 16 and 17, respectively, in which the head drivesignals are each converted from an analog form to a digital form. Then,the head drive signals are sent from the D/A converters 16 and 17 topower amplifiers 18 and 19, respectively, in which the head drivesignals are amplified. Then, the head drive signals are sent from thepower amplifiers 18 and 19 to the above-described head drive motors (notshown), respectively, which are driven by the head drive signals to movethe heads 11 and 12 along the radial directions of the optical disk 13,respectively, toward a selected track on the disk.

After the heads 11 and 12 have arrived at the selected track on the disk13, the head 11 serves to illuminate the selected track of the disk 13with laser light to perform erasure of information from or record ofinformation on the track, while the head 12 serves to illuminate theselected track of the disk 13 with laser light and then receive thelaser light reflected therefrom to perform reproduction of information.

On the other hand, during the seek movement of the head 11, it alsoserves to successively illuminate the tracks on the disk 13 with laserlight and to successively receive the laser light reflected from thetracks on the disk 13 in order to output information signals recorded onthe track. An address reproduction circuit 20 serves to take addressinformation signals each representing the address of one of the tracksfrom the information signals and output them to the CPU 14. Further, theinformation signals from the head 11 are wave-form shaped by a wave-formshaping circuit 21 into track count signals (see FIG. 9) which are thencounted by a counter 22, and then, an output signal representing thenumber of signals counted is sent from the counter 22 to the CPU 14.

Similarly, an address reproduction circuit 23 serves to take addressinformation signals each representing the address of one of the tracksfrom the information signals and output them to the CPU 14. Further, theinformation signals from the head 12 are wave-form shaped by a wave-formshaping circuit 24 into track count signals which are then counted bythe counter 22, and then, an output signal representing the number ofsignals counted is sent to the CPU 14.

FIGS. 10 to 12 show flowcharts for explaining a program to be executedby the CPU 14.

Referring first to FIG. 10, there is shown a main routine of the programin which the difference XE between two addresses of tracks at which theheads 11 and 12 take positions, respectively, is repeatedly obtained onthe basis of track address signals ADR1 and ADR2 supplied from theaddress reproduction circuit 20 and 23, and the difference XE obtainedis also repeatedly compared with a certain value CONST until XE becomesgreater than CONST. When XE becomes greater than CONST, the CPU 14generates head drive signals which are then sent to the D/A converter17. In this way, the head 12 is moved at the distance XE so that boththe heads 11 and 12 can be positioned at the same track on the disk 13.Therefore, the track is followed with the heads 11 and 12 until the CPU14 receives any access command CMD from the host computer 15.

When a seek command SEEK from the host computer 15 is received in theCPU 14, i.e., CMD=SEEK, as shown in FIG. 11, the number N of tracksbetween the address number N₁ of a target or selected track which isobtained by the seek command and the address number N₀ of the presenttrack at which the head 11 takes its position and which is obtained bythe track address signal ADR1 is obtained by N=N₁ -N₀. In the next seekroutine SEEK(N, MODE), the heads 11 and 12 are moved at the number N oftracks obtained.

In this embodiment, seek routine SEEK(N, MODE) includes three modes,i.e., a seek routine SEEK(N, 0) of mode-0, a seek routine SEEK(N, 1) ofmode-1, and a seek routine SEEK(N, 2) of mode-2. In the seek routineSEEK(N, 0) of mode-0, the heads 11 and 12 are moved at the number N oftracks on the basis of the head drive signals sent from the CPU 14 tothe D/A converters 16 and 17, respectively. On the other hand, in theseek routine SEEK(N, 1) of mode-1, only the head 11 is moved at thenumber N of tracks on the basis of the head drive signal sent from theCPU 14 to the D/A converter 16, while in the seek routine SEEK(N, 2) ofmode-2, only the head 12 is moved at the number N of tracks on the basisof the head drive signal sent from the CPU 14 to the D/A converter 17.

More specifically, for example, in the seek routine SEEK(N, 0) ofmode-0, as shown in FIG. 12, the above-described driven distance ratio βis first calculated from N, according to the relationship between x andβ in the optimum or simplified mode shown in Fog. 7. Thereafter,switching points n₁, n₂, and n₃ of driving current for the heads 11 and12 are calculated from the following equations.

    N=n.sub.1 +n.sub.2 +n.sub.3

    n.sub.1 =β N/2

    n.sub.2 =(1-β)N

    n.sub.3 =β N/2

Next, the CPU 14 reset the counters 22 and 25 and sends a head drivesignal ACC to the D/A converter 16, and thus the head 11 is acceleratedat a constant acceleration α until the count value CNT1 of the counter22 becomes equal to n₁. Also the CPU 14 sends a head drive signal ACC tothe D/A converter 17, and thus the head 12 is accelerated at a constantacceleration α after the count value CNT1 of the counter 22 has becomeequal to n₁ until the count value CNT1 thereof becomes equal to n₁ +n₂.Further, the CPU 14 sends a head drive signal -ACC to the D/A converter16, and thus the head 11 is decelerated at a constant acceleration αuntil the count value CNT1 of the counter 22 becomes equal to N.

According to the above-described embodiment, as shown in FIG. 13, theoptical disk system makes it possible to drive the heads 11 and 12 inthe form of triangular or trapezoidal pattern without overlapping thedriving currents i for the heads 11 and 12 with each other. Therefore,the system requires electric power consumption for the seek movement ofonly one of heads during seek movements of the heads 11 and 12 due toalternate power supply the respective heads 11 and 12. Moreover, sincethe head 12 starts to move during movement of the head 11 under theabove driving condition, that is, the seek motions of the heads 11 and12 are overlapped in time with each other, it becomes possible to speedup the time of seek-ending of the head 12 and thereby to shorten anoverall seek time of the heads 11 and 12.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives andmodifications will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to include allsuch alternatives and modifications as fall within the spirit and scopeof the appended claims.

What is claimed is:
 1. An optical disk apparatus comprising:a pluralityof heads for performing erasure of information from, record ofinformation on, and reproduction of information from an optical disk,each of said heads being movable in transverse to tracks on the opticaldisk for seek movement; means for driving the heads so as to move theheads to a target track on the disk, said driving means being adapted toapply for the seek movements of the heads a first drive force decided toaccelerate or decelerate each of the heads at a substantially constantacceleration and a second drive force which is smaller than the firstdrive force and decided to move each of the heads at a substantiallyconstant speed to each of the heads in such a manner that when one ofthe heads is driven by the first drive force, the other heads can bedriven by the second drive force only.
 2. An optical disk apparatusaccording to claim 1, wherein a ratio of moving distance of each of theheads obtained by the first drive force to a seek distance thereof is sodecided that a time-lag between times of seek-ending of the headsremains substantially within a predetermined permissible range.
 3. Anoptical disk apparatus according to claim 1, wherein the seek movementsof the heads are partially overlapped in time to each other.
 4. Anoptical disk apparatus according to claim 1, wherein the seek movementsof the heads are started with a time-lag therebetween.
 5. An opticaldisk apparatus comprising:a plurality of heads for performing erasure ofinformation from, record of information on, and reproduction ofinformation from an optical disk, each of said heads being movable intransverse to tracks on the optical disk for seek movement; means fordriving the heads so as to move the heads to a target track on the disk,said driving means being adapted to apply for the seek movements of theheads a first drive force and a second drive force which is smaller thanthe first drive force to each of the heads in such a manner that whenone of the heads is driven by the first drive force, the other heads canbe driven by the second drive force only, wherein the seek movements ofthe heads are partially overlapped in time to each other.
 6. An opticaldisk apparatus according to claim 5, wherein the ratio of movingdistance of each of the heads obtained by the first drive force to aseek distance thereof is so decided that a time-lag between times ofseek-ending of the leads remains substantially within a predeterminedpermissible range.
 7. An optical disk apparatus according to claim 5,wherein the seek movements of the heads are started with a time-lagtherebetween.
 8. An optical disk apparatus according to claim 5, whereinthe first drive force is decided to accelerate or declerate each of theheads at a constant acceleration, while the second drive force isdecided to move each of the heads at a constant speed.
 9. An opticaldisk apparatus comprising:a plurality of heads for performing erasure ofinformation from, record of information on, and reproduction ofinformation from an optical disk, each of said heads being movable intransverse to tracks on the optical disk for seek movement; means fordriving the heads so as to move the heads to a target track on the disk,said driving means being adapted to apply for the seek movements of theheads a first drive force and a second drive force which is smaller thanthe first drive force to each of the heads in such a manner that whenone of the heads is driven by the first drive force, the other heads canbe driven by the second drive force only, wherein the seek movements ofthe heads are started with a time-lag therebetween.
 10. An optical diskapparatus according to claim 9, wherein the ratio of moving distance ofeach of the heads obtained by the first drive force to a seek distancethereof is so decided that a time-lag between times of seek-ending ofthe leads remains substantially within a predetermined permissiblerange.
 11. An optical disk apparatus according to claim 9, wherein theseek movements of the heads are partially overlapped in time to eachother.
 12. An optical disk apparatus according to claim 9, wherein thefirst drive force is decided to accelerate or decelerate each of theheads at a constant acceleration, while the second drive force isdecided to move each of the heads at a constant speed.